[OwenPriceSkelly]

Problem

Spotted while verifying #138: on a host with no uv on PATH, the shell command's bootstrap block runs $PYTHON -m pip install uv on every task, into the same shared location (the user site). pip installs aren't concurrency-safe, so concurrent tasks can observe — and try to execute — a half-written uv binary. Observed as .../bin/uv: Permission denied when 6 tasks bootstrapped at once. It's rarer in practice than the env-creation race (#138) because deployed endpoints usually have uv on PATH, but any uv-less host running concurrent tasks can hit it.

Fix

Same publish-by-rename pattern as #138:

• Bootstrap installs with pip install --target into a unique temp dir ($UV_BOOT.tmp.$(hostname).$$), then atomically renames it to a persistent per-user dir under GROUNDHOG_CACHE_BASE (.../uv-bootstrap-$(uname -m); the arch suffix keeps installs from colliding when the cache base is shared across architectures). Only a working install (-x bin/uv) is published; the loser of a publish race discards its copy and uses the winner's.
• Once published, later tasks skip pip entirely — previously every task re-ran pip install uv even when it was already installed.
• The uv.find_uv_bin() discovery is kept as a fallback (e.g. the uv PyPI package already installed some other way), and command -v uv short-circuits everything exactly as before.
• The GROUNDHOG_CACHE_BASE block moves above uv resolution, since the bootstrap dir lives under it. No other reordering.

Independent of #138 (different section of the template); both apply cleanly.

Verification

• Full suite passes (357 tests), plus new tests asserting the bootstrap installs to the private target dir, publishes by rename, discards on a lost race, and that the cache base is computed before uv resolution.
• End-to-end: ran the bootstrap section of a real rendered shell command 6× concurrently with uv stripped from PATH and a cold cache — all 6 succeeded and resolved a working uv 0.12.8 (previously this exact setup produced Permission denied), leaving exactly one published uv-bootstrap-arm64 dir and no temp leftovers. A subsequent warm run reused it without invoking pip.

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